This invention relates to a method of shaping the distal surface of a human femur using certain alignment guides and instruments to prepare that surface to receive a distal femoral prosthesis and also relates to certain apparatus used in that method.
Various types of instruments and methods have been developed to enable a surgeon to affix a distal femoral knee prosthesis to the human femur. Since the purpose for affixing such a prosthesis is to restore the patient's ability to walk after disease or other traumatic causes have impaired that ability, it is important that the prosthesis be attached to the femur in such a manner that it will approximate as closely as possible the natural condyles which the prosthesis is replacing. If the prosthesis is not properly affixed with respect to the femur, an unnatural gait or other complications can result.
It is a common practice to use the long central axis of the femur as a guide in determining the manner in which the distal femoral surfaces should be shaped to receive a properly aligned distal femoral prosthesis. Generally a pre-operative single, long anterior-posterior radiograph showing the shaft of the tibia and femur is made and the angle of the central long axis of the femur relative to the vertical axis of the body (physiological valgus, generally from 5.degree.-12.degree.) is visualized. That angle is then used as a reference when the distal femoral surface is shaped using various cutting instruments and guides. In one such method, a long axial alignment jig (rod) is employed which is positioned over the outside surface of the patient's leg in a position which the surgeon visually determines to correspond to the central long axis of the femur and the femur is shaped relative to the alignment of that rod. One example of the manner in which the distal femoral surface is shaped to receive a prosthesis using an external alignment rod is shown in "The HOWMEDICA.RTM. Universal.TM.Total Knee Instrument System", brochure no. H- 2026-1 1/82 15M B (1980) from Howmedica, Inc., Orthopaedics Division, Rutherford, NJ 07070 which is hereby incorporated by reference.
The external alignment rod has a disadvantage in that the surgeon is relying upon visual and tactile means for positioning in alignment rod since the patient's skin covers the major portion of the femur and screens it from view. Finding the central long axis of the femur of the obese person can present further difficulties.
The use of a relatively short femoral alignment rod in a method for shaping the distal femoral surface is shown in a brochure entitled "New Jersey Tricompartmental Total Knee Replacement Surgical Procedure by Frederick F. Buechel, M.D", 13 pages, issue date 1/1981, Form No. 1280-32, from DePuy Division, Boehringer Mannheim Corporation, Warsaw, Ind. 46580. Part of this procedure employs a femoral alignment rod to hold a femoral resection guide against the distal femoral surface after a drill bit is used to create a shaft for the femoral alignment rod. The shaft is drilled in such a manner that the drill bit and, later, the alignment rod rests inside the intramedullary canal against the posterior femoral cortex. After the anterior and posterior femoral articular surfaces are cut, the alignment pin and the femoral resection guide positioner are removed and an extension-tension femoral alignment guide employing an external alignment rod is employed to resect the inferior femoral articular surface to the desired degree of physiological valgus (5.degree.-12.degree.). That guide is removed and an oblique osteotomy guide, a portion of which is designed to fit into the shaft for the femoral alignment rod, is then employed to resect and drill prosthesis mounting holes in the distal femoral surface. The final femoral resection is accomplished with a recessing guide which fits over the oblique osteotomy guide.
In both of the above procedures, the alignment rods employed do not enable a surgeon to accurately follow the central long axis of the femur because the femur is not exposed to visual observation along its length. This can especially become a problem when the femur possesses a deformity which may somewhat alter its true central axis.